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charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 0EAE1A0004 X-Rspam-User: X-Rspamd-Server: rspam08 X-Stat-Signature: y1mzddjzw8x58rixbs4x5mucoz9kg3n4 X-HE-Tag: 1718216931-316742 X-HE-Meta: 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 0xcitIsP Ky6d0NS+ngD/jqR2FTWFZyZXe+eSRsiyRoobpR/5vk3ip0C+GPxLFeOEodhIAT17LMEPgtmRnGxPpHVB/OX6ViKY9jze22B1USfVeUqNazbcySw1AB5S1vHDDhHQoNjkAcUPSCVxwvbGUy0GInXxvuMK0kfFmfEaf+onEundrbjurhx5+n6BIOcB6NDuBOEOwf4PaNEu52Xtd6rj7BJ0oYV5J2A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Jun 12, 2024 at 11:16=E2=80=AFAM Takero Funaki wrote: > > 2024=E5=B9=B46=E6=9C=8812=E6=97=A5(=E6=B0=B4) 3:26 Nhat Pham : > > > > > As I have noted in v0, I think this is unnecessary and makes it more co= nfusing. > > > > Does spin_lock() ensure that compiler optimizations do not remove > memory access to an external variable? I think we need to use > READ_ONCE/WRITE_ONCE for shared variable access even under a spinlock. > For example, > https://elixir.bootlin.com/linux/latest/source/mm/mmu_notifier.c#L234 In this example, it seems like mmu_interval_set_seq() updates interval_sub->invalidate_seq locklessly using WRITE_ONCE(). I think this is why READ_ONCE() is required in that particular case. > > isn't this a common use case of READ_ONCE? > ```c > bool shared_flag =3D false; > spinlock_t flag_lock; > > void somefunc(void) { > for (;;) { > spin_lock(&flag_lock); > /* check external updates */ > if (READ_ONCE(shared_flag)) > break; > /* do something */ > spin_unlock(&flag_lock); > } > spin_unlock(&flag_lock); > } > ``` > Without READ_ONCE, the check can be extracted from the loop by optimizati= on. According to Documentation/memory-barriers.txt, lock acquiring functions are implicit memory barriers. Otherwise, the compiler would be able to pull any memory access outside of the lock critical section and locking wouldn't be reliable.